Liquid crystal display devices are commercially well-known. Furthermore, in recent years, mobile devices are used in increasingly wide purposes. As such mobile devices, smartphones with liquid crystal display devices are well known, for example. As to such liquid crystal display devices, improvement of display quality is in great demand to achieve higher definition, higher color purity, and higher brightness of the display. Furthermore, lower energy consumption is also in great demand to achieve a longer battery drive.
In order to satisfy the above contradictory demands for achieving the higher color purity, higher brightness, and lower power consumption at the same time, research and development of liquid crystal display devices using a pixel structure of four color subpixels: red, green, blue, and white (RGBW) are keen to substitute an ordinary pixel structure of three color subpixels: red, green, and blue (RGB).
However, when using a so-called RGBW stripe pixel structure (in which columns of four subpixels of RGBW extending linearly are arranged in a row direction), each pixel has a slender shape which causes a significant decrease in display uniformity. To solve such a problem of the decrease in display quality, a so-called RGBW square pixel structure (in which four subpixels of RGBW are arranged in a square) is under development.
Here, comparing the RGBW square pixel structure to the RGBW stripe pixel structure, the number of subpixels arranged in each column of the RGBW square pixel structure is twice that of the RGBW stripe pixel structure. That is, the number of scanning lines of the RGBW square pixel structure is twice as much, too. What should be noted here is a writing time. The writing time of image signals from signal lines to subpixels varies depending on the number of scanning lines, and the time must be shortened if the number of scanning lines increases. The resolution in the horizontal direction can be improved by simply increasing the number of signal lines and it has no effect on the writing time. However, when higher definition of display performance and greater frame frequency are aimed, reduction of the writing time of image signals is inevitable. As a result, a writing time of image signals will become insufficient and energy consumption in a driving circuit will increase significantly due to the increase of driving frequency.
In consideration of the above, there is a technique under development which provides one scanning line per row of RGBW square pixels while providing two signal lines per column of RGBW square pixels. That is, four subpixels of an RGBW square pixel share a single scanning line. With this technique, even when the RGBW square pixel structure is used and the driving frequency is increased, a sufficient writing time of image signals can be secured. Furthermore, the energy consumption in a driving circuit can be suppressed (that is, lower power consumption can be achieved).
Incidentally, if reflection-type liquid crystal display devices are manufactured using pixels each including a plurality of subpixels (for example, RGBW square pixels), the arrangement of the scanning line to be shared with the subpixels is important.
In conventional reflection-type liquid crystal display devices, the scanning line in each square pixel unit has been laid under reflecting electrodes of either upper two subpixels or lower two subpixels in such a manner that the scanning line and the reflecting electrodes overlap. However, in this structure, the scanning line and the reflecting electrodes generate a coupling capacitance at the upper two subpixels and a coupling capacitance at the lower two subpixels those are different from each other and this causes display quality deterioration. As a possible approach to solve this problem, the scanning line may be formed at a gap in a boundary between the upper subpixels and the lower subpixels. However, in this approach, the line which does not contribute to the display itself is largely exposed from a gap between reflecting electrodes and will still cause the display quality deterioration.